1. Field of the Invention
This invention relates to controllers for industrial processes and particularly to the type of controller that provides so-called proportional speed floating control. The invention is particularly concerned with an electronic controller that is operative in response to signals representative of a process condition to produce a bi-directional low level direct current (d-c) output that may be utilized to regulate the direction and speed of operation of an electric motor valve actuator for a final control element, without valve position feedback, to maintain an industrial process at a desired value. The controller is particularly suitable for controlling industrial processes having short time constants.
2. Description of the Prior Art
Proportional speed floating control is a type of control action in which the direction and rate of adjustment of the final control element, such as a valve, damper, etc., is proportional to the direction and extent of deviation, from a desired set point, of the process variable that is being controlled. The final control element is said to "float" since the position of adjustment can be anywhere within the operating range when the deviation is zero.
Proportional speed floating controllers accept low level direct current voltage or current input signals from primary sensors responsive to process variables to provide, for example, rate of flow control, by means of motor driven valves, of water, sewage, sludge, slurry, and other process fluids. Such controllers compare the signal from the flow or other process variable transmitter with a set point signal representing a desired flow rate, which set point signal may be either locally or remotely generated. When a difference appears between the actual and the desired process variable value, the controller provides output signals that control the supply of energizing current to a reversible electrical motor for rotation at a speed and in a direction to operate a control valve as required to restore the process variable to the desired value. The basic control mode is proportional velocity. That is to say, the output signals of the controller are speed signals that are proportional to deviation. These speed signals are integrated by the electrical motor as the latter drives to the correct valve position.
Controllers of this type may be employed to directly control the operation of a light-duty motorized valve having, for example, a rating of one ampere or less. In modern industrial process control systems, however, heavy-duty motorized valves having much higher power requirements are common. In order to enable such controllers to control the operation of such heavy-duty motorized valves, a power relay is employed therewith to control the required heavy current and or high voltage to the motorized valve. Power relays utilizing semi-conductor switches of the type normally referred to as an SCR, triac or thyristor have been employed because of their heavy current and high voltage handling capabilities. Such power relays are generally mounted adjacent the motorized valve at a location remote from the controller, with the controller output signals being transmitted to the power relay over a plurality of relatively high voltage transmission circuits.
The control of such remotely located power relays by electronic controllers has had certain drawbacks that have added substantially to the cost of achieving acceptable performance and reliability. These drawbacks have resulted from the fact that measuring low-level voltages and currents in circuits that are not referenced to the same potential can be rather difficult. A primary reason for such difficulty is that both the controller and power relay must be grounded, and when remotely located from each other one is grounded at one location and the other is grounded at another location which may be at a different potential than the ground at the first location. Another factor that has contributed to increased costs and decreased performance and reliability is the difficulty in transmitting without phase shift and signal degeneration, that is, at a one-to-one time rate, the output signals from the controller to the relay. This difficulty has been due to the effects of capacitance and inductance in the transmission circuits connecting the controller to the power relay.